Recently, there has been increased awareness of the potential impact that greenhouse gases have on the environment. This increased awareness has resulted in the development of new technologies to lower emissions and/or to capture or store the CO2 found in such emissions.
Refinery process furnaces using air as an oxidant typically produce 8 to 10 moles of nitrogen for every mole of methane consumed.
Current technologies for separating CO2 from nitrogen in flue gas include amine absorber towers and/or membrane separation units. Unfortunately, it is not very cost-effective to separate carbon dioxide from flue gas using these technologies. The flow streams in the separation units have to be cooled, dried, and the pressure of the flow streams has to be raised significantly in order to achieve reasonable carbon dioxide removal efficiency. If such a separation is possible, the separation is costly and can be labor and/or equipment intensive.
Some have considered replacing air with pure oxygen as the oxidant when operating such furnaces. Pure oxygen generally is free of nitrogen and thus provides nitrogen free combustion.
Unfortunately, the cost of supplying and/or transporting pure oxygen to process heaters is relatively high. For example, pure oxygen may be supplied by Air Separation Units (ASU), Vacuum Swing Adsorption (VSA) units, nitrogen separating polymeric membranes, and/or as cryogenic liquid oxygen transported by trucks. The cost of supplying pure oxygen to the process would depend, for example, on the quantity of oxygen required, the purity of oxygen required, and the production and transportation costs for the pure oxygen.
Pure oxygen also lacks diluents, such as nitrogen, to reduce flame temperatures. As a result, combustion of a fuel comprising pure oxygen as the oxidant produces a flame that is very hot and very intense. The adiabatic flame temperature of a natural gas/pure oxygen flame may reach about 5000° F. Such temperatures far exceed the process heating temperatures of about 2000° F. generally used in refinery processes. Further, greater than 2000° F. process temperatures reduce the life of process tubes due to high temperature oxidation and reduction in creep strength of most metals. Because of the high flame temperature of oxy-fuel combustion and the need for using high temperature exotic alloys for process tube construction, the process industry has been skeptical of using pure oxygen as an oxidant in refinery process furnaces.
Efficient methods and oxidants are needed to reduce greenhouse gas emissions while maintaining a safe flame temperature during the operation of furnaces and burners.